This invention relates to integrated circuits and, more specifically, to an optical semiconductor device having, integrally formed therein, a photo diode and a bipolar transistor.
A conventional optical semiconductor device is disclosed in, for example, Japanese Laid-open Patent Publication No. 1-205564. With reference to FIG. 11, P-type semiconductor substrate 1 has epitaxially grown thereon a P-type epitaxial layer 2. An N-type epitaxial layer 3 is epitaxially grown on P-type epitaxial layer 2. A plurality of P.sup.+ -type separating areas 4 separate N-type epitaxial layer 3, and contiguous upper portions of P-type epitaxial layer 2 into isolated islands. A first island forms a photo diode 9. A second island forms an NPN transistor 10.
Photo diode 9 includes an N.sup.+ -type diffusion area 5 in its upper surface. An N.sup.+ -type buried layer 6 spans a substantial part of the interface between epitaxial layers 2 and 3 in the island forming NPN transistor 10. The portion of N-type epitaxial layer 3 in NPN transistor 10 functions as the collector thereof. A P-type base area 7 is disposed in an upper surface of NPN transistor 10. An N.sup.+ -type emitter area 8 is disposed in an upper surface of P-type base area 7. An N.sup.+ -type collector contact area 12 is disposed in an upper surface of NPN transistor 10 outside P-type base area 7.
Photo diode 9 is biased to form a PN junction between P-type epitaxial layer 2 and N-type epitaxial layer 3. N.sup.+ -type diffusion area 5 serves as a cathode of photo diode 9. P.sup.+ -type separating area 4 serves as an anode of photo diode 9.
An accelerated electric field is formed in NPN transistor 10 by an auto-doped layer 11 in P-type epitaxial layer 2 which becomes autodoped by diffusion of P-type carriers from substrate 1 during epitaxial growth and heat treatment. Auto-doped layer 11 retards the movement of carriers originating below the depletion region.
To obtain a high speed response of photo diode 9, the depletion region is widened to restrain the movement of carriers occurring outside the depletion region. In the structure of the prior-art device in FIG. 11, auto-doped layer 11 overlaps P-type epitaxial layer 2. This overlap results in an increased concentration of impurities and an enlargement of the depletion region.
Epitaxial growth requires processing in a closed vessel into which gasses are fed to grow the desired layers, and to introduce the desired amount of impurities. During growth of P-type epitaxial layer 2, the closed vessel becomes contaminated with the P-type impurities. If an attempt is made to grow N-type epitaxial layer 3 in the same closed vessel used to grow P-type epitaxial layer 2, the P-type contaminants in the vessel enter N-type epitaxial layer 3 in such amounts that it is difficult or impossible to attain the desired properties in N-type epitaxial layer 3.
As a consequence of the contamination of the vessel by P-type impurities during epitaxial growth of P-type epitaxial layer 2, the device must be removed physically from the vessel after P-type epitaxial layer 2 is formed, and placed in a clean vessel for growth of N-type epitaxial layer 3. The requirement for removing and reinstalling the workpiece during processing interferes with production since the same epitaxial growing device cannot be used for all steps to produce the prior-art device.